1. Field of the Invention
The present invention relates generally to ion implantation of plastic orthopaedic implants and, more particularly, to a load-bearing orthopaedic joint implant partially or wholly formed of UHMWPE material and to a process for subjecting a load-bearing surface thereof to ion implantation so as to improve its surface characteristics.
2. The Prior Art
The present invention is an improvement over a previous invention of the applicants disclosed in U.S. Pat. No. 4,743,493, ION IMPLANTATION OF PLASTICS, granted May 10, 1988 and assigned to a common assignee, Spire Corporation of Bedford, Mass.
As stated in said U.S. Pat. No. 4,743,493, ion implementation is a method that was originally developed in the semiconductor industry to effect precise doping of silicon wafers with impurities. From there, the method spread to the ion implantation of metals and compounds, see Treatise on Materials Science and Technology, Vol. 18, "Ion Implantation," 1980, Academic Press, Inc. See also a copending application, assigned to the common assignee herein, entitled "Ion Implantation of Titanium Workpieces Without Surface Discoloration", Serial No. 861,845, filed May 12, 1986.
In the specific area of ion implantation of plastics, in the beginning most of the interest and research has focused on advantageously changing the electronic and transport properties of a variety of polymers that are normally insulating. By using masks, for instance, conducting paths in an insulating medium are fabricated. See the article authored by M.S. Dresselhaus et al of M.I.T. "Ion Implantation of Polymers," Mat. Res. Soc. Symp. Proc.," Vol. 27 (1984), pp. 413-422. Other workers in the field have studied the reaction of iodine ions with solid alkanes and the effect of carbon ions on benzene. See 7th Int. Hot Atom Chem. Symp. (1973) p. 19, and R. M. Lemmon, id., p. 20. Similar work in organometallic compounds, such as metal carbonyls, has indicated that energetic ions, such as copper and chromium, as well as rare-earth ions, interact with such compounds and that about half of the incoming ions become incorporated in the organometallic chain. See G. K. Wolf, "Ion Bombardment Chemistry," Nucl. Instrum. Methods 139 (1976) 147.
Our said previous invention, disclosed in said U.S. Pat. No. 4,743,493, primarily and principally dealt with the ion implantion of plastics, such as used as transparencies in aircraft, safety glasses and face masks, eyeglasses and contact lenses, or non-transparent plastics, such as used in bearings or safety helmets, so as to enhance their surface hardness or their resistance to chemical attack.
In biomedical applications, polyethylenes find important uses, inter alia, as acetabular cups in joint replacements, against which a metal part, lately formed of a titanium alloy or a CoCr alloy, known as F-75 alloy, is designed to bear. As known, the material of choice for the vast majority of total joint replacements is Ultrahigh Molecular Weight Polyethylene (UHMWPE) articulating against a mating surface formed of a Ti-6A1-4V alloy. Note R.M. Rose et al., "Exploratory Investigations on the Structure Dependence of the Wear Resistance of Polyethylene," Wear, 77 (1982), pp. 89-104; R.M. Rose et al., "On the Pressure Dependence of the Wear of Ultrahigh Molecular Weight Polyethylene," Wear, 92 (1983), pp. 99-111; R.M. Rose et al., "Radiation Sterilization and the Wear Rate of Polyethylene," Journal of Orthopaedic Research, 2: 393-400; and I.C. Clarke et al., "Wear of Ti-6A1-4V Implant Alloy and Ultrahigh Molecular Weight Polyethylene Combinations," Titanium Alloys in Surgical Implants. ASTM, STP 796 (1983), p. 136.
Until recently, difficulties of bone attachment and of body rejection have overshadowed and eclipsed wear rate in joint replacements. Advancements in technology have, however, effectively dealt with both bone attachment and body rejection. These improvements have, therefore, refocussed attention on the wear rate of UHMWPE, being now the primary cause of premature failure of an implanted joint prosthesis, requiring its premature replacement.